Macromol. Chem. Phys. 199,2547-2551 (1 998) 2541

Imaging systems by dye-sensitized photooxidation of oxazole groups attached to polymer backbones, 5a

Photooxidative amplification

Hiromitsu Ito2, Kunihiro Ichimura *' I Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,

* Materials Research Center, Toppan Printing Co., Ltd., 4-2-3 Takanodai-minami, Sugito, Saitama 345, Japan

(Received: June 19, 1997; revised: April 7, 1998)

Yokohama 226, Japan

SUMMARY Copolymers of 2-(3-methacryloyloxypropyl)-4,S-diphenyl- 1,3-0xazole with N-isopropylacryl- amide are used as positive-working photoresists. The change of solubility is induced by the dye-sensitized photooxidation of oxazole rings into the corresponding N, N-dibenzoylcarboxamide and subsequent develop- ment in aqueous ethanolamine solution. We propose here a novel procedure to result in a marked amplifica- tion of oxazole ring photooxidation by reacting the tricarboxamide photoproduct with an ionizable amino derivative, dyeing with an ionic photosensitizer, exposing to flood light and finally developing the photo- image.

Introduction Visible-light-sensitive polymers have been applied to various imaging systems coupled with laser light sources emitting light in visible regions. Their representative applications include the production of direct printing plates'), holographic recording2), three-dimensional fabri- cation3), laser-to-plate photo resist^^), and so on. In order to make the photopolymer systems applicable to laser- sensitive photoimage formation, considerable enhance- ment of their photospeed is highly required. Major part of polymeric materials sensitive to visible light consists of radical as well as cationic polymerization of monomers or oligomers initiated by dye sensitizers5). An alternative polymeric system sensitive to visible light is based on the so-called chemically amplified photoresist systems6), whereas photopolymerization systems including photoin- duced pumping cycles to generate photoinitiator were reported7). Another system employs the dye-sensitized photooxidation of olefinic polymers by singlet oxygen to form hydroperoxide residues at polymer backbone^^,^).

We reported previously that oxazole rings attached to polymer backbones are converted efficiently into the cor- responding N,N-dibenzoylcarboxamide derivative by photooxidation using triplet dye sensitizers. The photooxi- dation is induced by irradiation with light of wavelengths in a range from 400 nm to 700 nm and alters the solubility of thin films of polymers with photooxidized oxazole side chains upon treatment with an aqueous amine solution to provide positive-working photoresists'O). These visible-

* Part 4: cf. ref.")

light-sensitive polymers were applied to fabricate holo- graphic gratings"), and the enhancement of photosensitiv- ity has been required to extend the applicability. We report here a novel procedure to result in the marked amplifica- tion of the photooxidation of oxazole rings, leading to improve their photosensitivity.

Experimental part

Materials A vinyl monomer including an oxazole moiety, 2-(3-meth- acryloyloxypropyl)-4,5-diphenyl-1,3-oxazole (OM), was synthesized according to our previous paperlo). The monomer was subjected immediately to radical polymerization because of its poor storage stability. N-Isopropylacrylamide (IPAm) was recrystallized from a 1 : 3 (v/v) mixture of ben- zene and hexane.

Polymerization

Radical copolymerization of OM (1,53 g; 4,40 mmol) with IPAm (1 ,SO g; 13,3 mmol) in benzene was performed accord- ing to our report'O). The composition of a copolymer poly(0M-co-IPAm) with Mw = 12,2 x lo4 and MJM,, = 2,41 was determined by UV spectroscopy. UV-visible spectra were determined by HITACHI 320 photospectrometer. The molar fraction of OM in the copolymer was 0,20. The mole- cular weight of the copolymer was determined by gel-per- meation chromatography using chloroform as an eluent and polystyrene as a calibration standard.

Macromol. Chem. Phys. 199, No. 11 0 WILEY-VCH Vedag GmbH, D-6945 1 Weinheim 1998 1022-1352/98/1111-2547$17.50+.50/0

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H. Ito, K. Ichimura

Light sources Monochromatic light at 410 nm with intensity of 0,029 mW/ cm2 was obtained from a 500 W high pressure Hg arc passed through a glass filter (Toshiba UV-39) and an interference filter (Toshiba KL-40) and used for exposure experiments with low doses. High dose irradiation with light at 419 nm (43 mW/cmz), 580 nm (2,7 mW/cm2) or 659 nm (2,2 mW/ cm2) was carried out with a JASCO Spectroirradiator CRM- FA. The light intensity was determined by the use of ADVANTEST OPTICAL SENSOR TQ8210. Photoirradi- ation was carried out at room temperature in the air.

Scheme 1: Principle of the photoimaging system used

Amplification procedure A 5 wt.-% solution of poly(0M-co-IPAm) in chloroform dis- solving 0,38 mol-% (relative to OM unit) of meso-tetraphe- nylporphine (TPP) was spin-cast on quartz plates, and these films were dried for 20 min at 80°C. Film thickness was ca.13 pm. After irradiation (monochromatic light at 410 nm or 419 nm), films were dipped for 5 min in an aqueous solu- tion containing 30 wt.-% ethanol, 2,8 wt.-% taurine and 2,2 wt.-% triethylamine, followed by immersion in diethyl ether to extract TPP. They were subsequently immersed in a 7: 3 (w/w) mixture of water and ethanol containing 3,l x wt.-% of methylene blue. After rinsing with aqueous ethanol and air-drying of the films, they were subjected to exposure to 659 nm light with intensity of 2,2 mW/cm2 from the Spec- troirradiator. The consumption of the oxazole rings and film thickness in dried films were estimated by UV spectroscopy.

Results and discussion

Concept of photooxidative amplification

The copolymer with OM molar fraction of 0,2 is not sol- uble in water, since OM units act as a dissolution inhibi- tor. Dye-sensitized photooxidation brings about the trans- formation of diphenyloxazole residues into N,N-diben- zoylcarboxamide which reacts with ethanolamine to give hydrophilic N-hydroxyethylcarboxamide units (Scheme 1). As a result, photooxidized areas of polymer films become soluble in water. This is the principle of the pres- ent photoimaging system. Fig. 1 shows the sensitivity curve of the film of poly(0M-co-IPAm) (OMAPAm = 1/4) doped with TPP as a function of irradiation doses of 410 nm light, followed by treatment of irradiated films with an aqueous solution of ethanolamine. Exposure doses of approximately 400 mJ/cm2 were necessary to achieve complete dissolution. The sensitivity was much lower than that of other systems involving dye-sensitized photooxidation and radical polyrneri~ation'-~).

Taking into account that oxazole side chains act as a dissolution inhibitor in this system, the relationship between the level of photooxidization of oxazole rings and the solubility of photoirradiated films in water was

"-3- -I-- ml-

e HzNCYCYOH " 0 ,( O 'N'Ph - 0

TPP

PhHPh Ph OH

determined. The conversion of the oxazole after photoir- radiation of films was estimated by means of UV absorp- tion spectra. As illustrated in Fig. 2, films become soluble in an aqueous solution of ethanolamine at a conversion of 85% or more. Attempts were made to reduce exposure doses required for the dissolution of films by lowering the level of diphenyloxazole unit. But problems arose from the fact that films with a lower level of oxazole of 3,2 mol-% or less in copolymers with IPAm were severely damaged due to swelling by the treatment with an aqueous ethanolamine solution for development"). These results have led us to achieve the following photo- oxidative amplification.

Our concept to enhance the photosensitivity is based on dual photooxidation, as illustrated in Scheme 2. The first step consists in the partial photooxidation of oxazole rings for latent image formation by TPP-sensitization to give N,N-dibenzoylcarbamide residues. After the removal of TPP, the irradiated film is treated with taurine in the presence of triethylamine to introduce sulfonic groups in photoirradiated areas. The film is subsequently dyed with a cationic dye sensitizer such as methylene blue (MB) to generate singlet oxygen upon visible light irradiation of a whole area with high exposure doses to result in sufficient photooxidation of unreacted oxazole residues. Finally, the film is treated with an aqueous ethanolamine solution to convert the oxidized hydrophobic residues into 2-hydro- xyethyl groups, leading to the solubilization of exposed areas in water. When N,N-dimethylethylenediamine is

Imaging systems by dye-sensitized photooxidation of oxazole groups attached to polymer backbones, 5

Scheme 2: Mechanism of photooxidative amplification

(1) NHzCHzCH2SO3Na I NEt3

/ (2) Methylene blue (MB)

.Nvv-

-1- N' TPP 0 'N'Ph Ho 02 hv-1 P A 0

Ph Ph

(1) NH2CHzCHzNMe2 t

(2)Rose bengal (RB) u' 0 2 1 o r 2 hv-2

2549

-1- 0 YH

\1 SOB MB+

1 -

0.20- I 0.5

0,15-

0 , l O -

0.05 -

I

600 620 640 660 680 700

Wavelength in nm Conversion in %

Fig. 2. Solubility of exposed film in a 5 wt.-% aqueous 2-ami- noethanol solution as a function of the conversion of oxazole units. Irradiation wavelength: 410 nm, initial film thickness: 1,5 pm 50 mJ/cm2

Fig. 3. Visible absorption spectra of poly(0M-co-IPAm) after irradiation in the presence of TPP and subsequent dyeing with MB. Irradiation doses of 410 nm light were in the range of 0.5 to

employed instead of taurine for the reaction with a par- tially photooxidized film to attach 2-dimethylaminoethyl residues in the side chains, the film is subsequently dyed with an anionic dye sensitizer such as rose bengal (RB), followed by visible light irradiation for the photooxida- tive amplification process.

Methylene blue as amplification sensitizer In order to verify the photooxidative amplification of a polymer with 4,5-diphenyloxazole side chains, a thin film of the copolymer poly(0M-co-IPAm) was first irradiated with 419 nm or 410 nm light to excite the Soret band of TPP as a sensitizer doped in the film. The light of 410 nm was used only when a film was irradiated with 0,5 mJ/ cm2 exposure dose. The film was subsequently treated

with diethyl ether to remove TPP away and dipped in an ethanol solution of taurine. The photoinduced triacyl- amino moieties reacted easily with amino group of taur- ine at the carbonyl group as shown in Scheme 2, resulting in the introduction of 2-sulfoethyl residues in the side chains. Triethylamine was added to the ethanol solution to accelerate the nucleophilic reaction. Both processes resulted in no deterioration of film properties. The film was then dyed with methylene blue in ethanol. Fig. 3 shows the absorption spectra of films irradiated with vari- ous exposure doses less than 50 mJ/cm2 and dyed with MB. Essentially no dyeing was observed for a non-irra- diated film, whereas the absorbances rise due to increas- ing MB binding with increasing exposure doses. As seen in Fig. 1, no marked reduction of film thickness is brought about by irradiation with an exposure dose of

2550

100

* 75

9 c .-

.C 0 1 50 0 u.

I

: 6 '' 25

0 I

I - 0.5 1.0 1.5 2,0 2.5 3.0

Exposure dose in J/cm

Fig. 4. Conversion of oxazole rings in poly(0M-co-IPAm) dyed with MB as a function of the second irradiation with 659 nm light of 2,2 mW/cmz. With respect to the first exposure doses and adsorbed quantity of MB, see Fig. 3. The first expo- sure doses; (m): 0, (0): 0.5, (A): 5 , (0): 25, and (0): 50 mJ/cm2

50 mJ/cm2, indicating that the photooxidation of oxazole rings occurs only partially. These results imply that the nucleophilic reaction with taurine and the subsequent dyeing with MB proceed efficiently.

Films of the copolymer dyed with MB after 419 nm or 410 nm excitation at various exposure doses were exposed to 659 nm light for the photooxidative amplifica- tion and subsequently subjected to the estimation of the consumption of oxazole moieties. The results are sum- marized in Fig. 4. No decrease in the level of the hetero- cycle was observed for the films which were not exposed to the light for TPP sensitization and consequently not dyed with MB. The rates of the disappearance of the oxa- zole increase with the increase in exposure doses for MB excitation. When exposure doses for the TPP-sensitiza- tion exceed 25 mJ/cm2 or more, more than 90% of oxa- zole moieties are converted into the photooxidized form. As expected from the results presented in Fig. 2, the mod- ified films can be dissolved in an aqueous solution of ethanolamine to give photoimages. Interestingly, MB- sensitized photooxidation proceeded effectively for films exposed to a dose of 0,5 mJ/cm2 though the absorbance of MB was quite minute. However, the conversion did not reach the critical value of ca. 85% required for solubiliz- ing the film in the given experimental conditions. One can anticipate that iterating the photooxidative amplifica- tion procedure can improve significantly the copolymer photosensitivity.

Rose bengal as amplification sensitizer In order to extend the photooxidative amplification, latent photoimages formed by the TPP-sensitized photooxida-

0,015

0.01 0 0

Y

H. Ito, K. Ichimura

500 520 540 560 580 Wavelength in nm

Fig. 5. Visible absorption spectra of poly(0M-co-IPAm) films after irradiation with 410 nm light in the presence of TPP and dyeing with RB

'01

0 25 50 75 100

Exposure dose in J / cm

Fig. 6. Conversion of oxazole rings in poly(0M-co-IPAm) film dyed with RB as a function of the second irradiation with 580 nm light of 2.7 mJ/cm2. With respect to the first exposure dose and the adsorbed quantity of RB, see Fig. 5. The first exposure doses; (0): 0, and (0): 4 mJ/cm2

tion were modified with Rose Bengal (RB) as an anionic triplet sensitizer. For this purpose, TPP-photooxidized films were immersed in a solution of N,N-dimethylethyle- nediamine to introduce dyeable cationic sites to polymer backbones after the removal of TPP in a way similar to that for the MB dyeing. Fig. 5 shows spectra of poly(0M- co-IPAm) films before and after irradiation in the pres- ence of TPP and subsequent treatment with the diamine solution and RB solution. Although the absorbance due to RB of a film irradiated with 4 mJ/cm2 dose was only in the range of lo-', the exposure of the film to 580 nm light for the RB-sensitization leads to a gradual consumption of the oxazole rings, as presented in Fig. 6. When com- pared with the MB-sensitized amplification as given in

Imaging systems by dye-sensitized photooxidation of oxazole groups attached to polymer backbones, 5 255 1

Fig. 4, the conversion rate of the oxazole was two orders of magnitude lower. This may reflect the difference in the efficiency of photosensitization.

Conclusion A novel method to improve the photosensitivity of copo- lymers of N-isopropylacrylamide with diphenyloxazole side chains has been exploited. The procedure consists of the following steps: (1) dye-sensitized photooxidation of the oxazole moieties in a film for latent image formation (first irradiation), (2) the removal of the sensitizer dye for the image formation, (3) the nucleophilic reaction of photoformed N,N-dibenzoylcarboxamide residues with dyeable amines, (4) dyeing with an ionic sensitizer dye for photooxidative amplification, (5) further photooxida- tion of oxazole moieties at the latent areas by exposure to a light which the ionic sensitizer absorb (second irradia- tion) and (6) development with an aqueous solution of ethanolamine to form photoimages.

The characteristic features of the present photosensitive polymeric systems are summarized as follows. (1) The exposure dose of ca. 400 mJ/cm2 required for image for- mation with the copolymer poly(0M-co-IPAm) is con- siderably reduced using methylene blue as a sensitizer for the photooxidative amplification to realize an amplifica- tion degree of ca. lo3. (2) The present system displays excellent storage stability, and the photooxidative ampli- fication process can be carried out with room light after

The amplification procedure consisting of many steps is not fully satisfying. But both the dyeing and develop- ment processes employ aqueous solutions except for the removal of TPP, so that the replacement of the sensitizer by a water-soluble sensitizer dye should be a necessary condition for practical applications.

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